1. Field of the Invention
The invention relates to an aqueous polyacrylate dispersion for the preparation of a coating to serve as a finishing varnish coat on a polyvinyl chloride article surface.
2. Discussion of the Background
After polyvinyl chloride (PVC) is made into semifinished or finished products, these objects are usually given an additional surface treatment. This surface treatment, which at its simplest may take the form of additional coating with a varnish, may be done both for decorative and functional reasons. For decorative effect, for example, the degree of luster of the surfaces of the PVC objects can be altered, and in the process the stability of the PVC against chemical and physical degradation can be improved.
Even in the case of synthetic leathers, which are essentially synthesized from fibrous materials and polymers as laminating raw materials, a significant portion is laminated with plasticized PVC. The surfaces of synthetic leather laminates are treated with a thin varnish, known as a finishing varnish.
In addition to the opportunity it affords to apply a decorative design with a look of leather and to impart the feel of leather, the finishing varnish coat also serves a protective sealing function that prevents exudation of plasticizers. This prevents the PVC surfaces from becoming brittle and sticky, and dirt from adhering to the surface.
For the finishing (surface) varnish coat, various polymers are in use that produce the desired properties to a greater or lesser degree. The varnishes are applied to the PVC surfaces as polymer solutions. In addition to polyurethanes and polyamides, hard polyacrylates, i.e. polymers with relatively high proportions of polymethyl methacrylate, which are known to make varnishes resistant to weather and by ethanol, are used as finishing varnishes. As adhesion enhancers for the polyacrylate coating on PVC surfaces, adhesion-enhancing substances such as vinyl chloride/vinyl acetate copolymers are added.
For considerations of environmental protection and work safety, attempts have been made to replace varnishes containing solvents with aqueous systems. For this purpose, aqueous dispersions of polyacrylates have been suggested, which may if necessary contain other, unsaturated additives, and which are then hardened to a coating by cross-linking after application at higher temperatures, if necessary (Ullmann, Encyklopadie der Technischen Chemie, 4th Ed., Vol. 15, pp. 163-170).
DE-C 25 59 790, for example, claims a printer's lake on an aqueous base for the preparation of gravure printing colors for plastic strips, particularly for PVC films, using bonding agents on a base of a mixture of a polyacrylic acid ester dispersion and a polymethacrylic acid ester dispersion in a ratio by weight between 100:4 and 100:8. The esters of the polyacrylic acid and the polymethacrylic acid are of the type whose ester groups are preferably composed of straight-chain, branched or cyclic alkyl groups with 1 to 8, and preferably 1 to 4 carbon atoms. Particularly good results are obtained with methyl and ethyl esters.
These properties of the bonding agent on a polyacrylic acid ester and polymethacrylic acid ester dispersion base are in some cases sufficient for gravure printing colors for PVC films, and commercially available dispersions prepared according to the specifications given in DE-C 25 59 790 are found to be usable for the preparation of firmly adhering and relatively hard PVC finishing varnishes with, in some cases sufficient plasticizer sealing effect, but they are not resistant to ethanol and/or water.
DE-A 31 12 704 discloses an aqueous dispersion or emulsion for the preparation of a firmly-adhering coating on PVC surfaces that contains an acrylate polymer with a film-forming temperature of above 60.degree. C., at least one unsaturated polyester resin, 5 to 10% by weight methyl pyrrolidone and in some cases a melamine and/or urea-formaldehyde resin as well. This aqueous system must be hardened after application on the PVC surface by the addition of a cobalt accelerator and a water-soluble peroxide at a temperature of over 80.degree. C., preferably over 100.degree. C. The preferred suitable acrylate polymers are those that are prepared with methacrylic esters of lower aliphatic alcohols with from 1 to 3 carbon atoms and that are conventional and commercially available. For good adhesion of the coatings thus prepared onto PVC surfaces, the methyl pyrrolidone content is essential to the invention.
Because of the necessary hardening of the unsaturated polyester resin, which in itself makes the coating deficient in light-fast properties, the varnishing process is not simple. Problems arise both in terms of the shelf-life of such dispersions and in terms of the coatings because of their cross-linking. This makes it hard to varnish over them and prevents embossing of the finishing varnish because of a lack of themoplasticity.
DE-A 34 40 537 describes a process for coating plastic parts, particularly those made of hard PVC, that employs an aqueous dispersion of an aliphatic polyurethane with reactive carboxylic groups and a polyacrylate. Such polyurethane-containing systems yield coatings that are deficient in light-fastness and heat-resistance.